$($b$)$ A flat plate with length and width of $1\mathrm{.}5m,0\mathrm{.}25m$ has been placed

longitudinally in a stream of crude oil which flows with a velocity of $20\frac{m}{s}.$

The specific gravity and kinematic viscosity of the crude oil are $0\mathrm{.}9,1{10}^{-4}$

$\frac{m^2}{s}$ respectively. Determine the thickness of the boundary layer and the drag

force acting on the flat plate.